Didn’t the L.L. Bean catalog use the singular in the past, i.e., “This a versatile and durable pant?”
“Pair of underwear” sounds wrong.
“Pair of underpants” sounds right.
Shrugs
YMMV
Pedantically that’s wrong. “Many” is used for things that are counted. “Much” is used for things that are not counted, but would be measured. How much dirt is in that pile? How many pailfuls of dirt are there?
“How much underwear is in that pile? How many items of underwear are there?” You wouldn’t say “How many underwears do I have?”, would you?
Surely “underwear” is, grammatically, a mass noun, like “clothing” or “food”: the individual items may be counted, but the group of things that fit into the category named is treated (grammatically) as a mass.
This thread reminds me of a woman who always referred to a pair of bras. What a strange woman she was.
Hebrew has a special plural form for two of anything, which I take to mean it’s the equivalent of the English “a pair of _______”. It’s used for a lot more things than the English “a pair of …” is. For example, in English we don’t say “a pair of feet” or “a pair of hands”, but I think in Hebrew they would use the dual plural for all kinds of things like that.
They even use the dual plural for “tomorrow” to mean “the day after tomorrow”. (Literally, I suppose, “a pair of tomorrows”.) But strangely, they don’t use the dual plural for “yesterday” to mean “the day before yesterday”.
Not sure if it is relevant but “pants” were once two garments.
I medieval times you wore short pants (braes) that looked like boxers. Over that you wore two garments that consisted of, essentially, half a pair of pants. You put on one leg and tie it around your waste and then put on the other leg and do the same.
So, while they weren’t called pants then, there was a time when you actually wore a pair of pants.
Incredible. You guys get that a “pair” is “two”, right?
https://i.imgur.com/LRmK15u.png
Like, do I really have to explain this to you guys?
You sign up to dig up a zombie corpse just to link to a picture of a dictionary definition of a word that was not in contention?
Bold and other stuff mine
If you thought he meant two he would have said
Though that should really be
Incredible. You get that you could have just linked to the definition of the word “pair” without saving it as an image?
Like, do I really have to explain this to you?
Yes, please do. I’m not being facetious. I’m curious to hear your explanation in regards to underwear.
Unless you are in subjunctive mood. “If that pair of underwear were clean I could wear it; as it is, I can’t.”
What this old thread needs is an old song:
The answer to this age-old question is in the last line of the song.A “pair” of undies, but bra – which holds two items – is singular.
You’d think it would be a pair of bra.
Or something.
Underwear holds two butt cheeks, or two testicles if you’re a guy. 
Oddly, even using plurals, like pants, I’m less inclined to say pair if you’re wearing them than if they’re sitting around.
Don’t try to constrain me with your conventional underswear!
As mentioned several times in this thread, the garment we call “pants” derives from a garment originally made in two separate pieces, each covering one leg. The usage seems to have transferred intact to all terms for pants-like garments (underpants, boxers, briefs, Speedos, etc.).
The term brassiere, however, originally referred to a type of (under)shirt, and shirts/shirt-like objects do not take “pair of”, since they are primarily conceived of as unitary torso coverings, not paired arm coverings.
For another example, “Stays” (an older term for a corset), which could be constructed as two pieces laced or clasped together, also takes “a pair of”, even though it is not possible to wear only one half. (“Bodice”, and its older spelling “bodies”, also occasionally got “a pair of” in earlier eras.)
I’m surprised no one has mentioned “pair of glasses”; perhaps the separate-ness of the lenses makes this one more intuitive.
Note to self: men’s briefs are weirder than I thought, as they can only cover testicles but not butt cheeks.
According to Matt Groening, the word “underpants” is at least 15% funnier than the word “underwear.” [cite]